Recently, an office automation apparatus has rapidly been going digital and the demand for color image output has been increasing. This causes output devices, such as digital color copying machines of an electrophotographic type and color printers of an ink jet type or a thermal transfer type, to have been generally and widely used. For example, these output devices are used to output (i) image information supplied from an input device such as a digital camera or a scanner, or (ii) image information created on a computer.
Input images (original), read by an input device such as the scanner, have various printing methods. Incidentally, a layer structure of an image surface or a composition of a color material may cause a phenomenon called a color-unmatch. According to such a phenomenon, colors between which the human eye cannot discern are read and recognized by the scanner, etc., as image data having respective different colors. If such a phenomenon occurs, then the image to be outputted is reproduced in colors different from those of the input image. This causes the image quality to be deteriorated. In view of the circumstances, a variety of techniques have been proposed so as to solve this kind of problem.
Note that, the color-unmatch here occurs when “metamerism” is no longer satisfied. The “metamerism” indicates a state where colors, whose light sources have different spectral distributions from one another, are recognized as the same one. Further, the “metamerism” includes a state where the human eye cannot discern between two colors whose spectral reflectances are different from each other. The “metamerism” also includes a state where two input signals, sensed with respect to two different colors having different spectral reflectances from each other by a sensor, which has a different spectral sensitivity from that of the human eye, become unrecognizable.
This phenomenon is applied to output devices such as scanners or printers, and these output devices carry out color-matching by realizing the metamerism. However, there is a possibility that colors between which the human eye cannot discern are reproduced as different colors (color-unmatch), when such colors are read by the scanner and the respective image data are outputted by the output device. In this case, the metamerism is satisfied in the human eye, but the metamerism is not satisfied in the sensor of the scanner.
Generally, reading characteristics of a scanner are adjusted such that, in case of displaying images on a display device such as a display, most images recognized on the display device are the same ones as those actually recognized by the human eye. In other words, the reading characteristics of a scanner is adjusted such that the color-unmatch between the human eye and the sensor of the scanner is small. As such, the color-unmatch does not occur on most output originals (namely, the metemerism is satisfied between the scanner and the human eye). However, when a particular color material is used, the phenomenon (phenomenon in which the metamerism is not satisfied) occurs as long as the human eye and the sensor of the scanner have different spectral sensitivities from each other. In the present specification, the phenomenon in which the metamerism is not satisfied is defined as the “color-unmatch”.
As a method for preventing the color-unmatch which occurs when the metamerism is not satisfied, there is an image processing method disclosed in Japanese Unexamined Patent Publication No. 2002-94814 (Tokukai 2002-94814, published on Mar. 29, 2002). According to the image processing method of Tokukai 2002-94814, an original is read under at least two types of different reading conditions. Then, it is judged whether or not target pixels, which have been judged to have a predetermined color under one of the reading conditions (first reading), change to another predetermined color when reading with respect to the target pixels under the other reading condition (second reading).
The number of pixels that turned to the above another predetermined color during the second reading is counted and compared with a value. An identification of a type of the original is made in accordance with the result thus compared (whether the original is of photographic, electrophotographic, or inkjet). Then, a color correction coefficient is set in accordance with a determination result regarding the original. Note that the two different reading conditions of the first and second readings are realized by (1) using scanners respectively having different CCD characteristics, (2) using or not using an infrared cut filter, or (3) switching light sources, or the like.
Another technique for preventing the color-unmatch which occurs when the metamerism is not satisfied, may be contrived. In such another technique, the spectral reflectance is measured or estimated. This kind of technique is disclosed in, for example, Japanese Unexamined Patent Publication No. 2001-86354 (Tokukai 2001-86354, published on Mar. 30, 2001). According to the technique for estimating the spectral reflectance, a multiband camera, which takes a picture, for example, through eight filters respectively having different transmission wavelength bands, is used to shoot an object so as to acquire a plurality of spectral images respectively indicating color information of the object for each wavelength. Then, the spectral reflectance of the object is estimated in accordance with the spectral images. An image, which is recognized as an image which picture is taken under a desired light source, is acquired in accordance with the estimated spectral reflectance. With this technique, it is possible to acquire a high-accurate image without damaging the color reproducibility of the object, even if a picture of the object having many colors, such as a painting, is taken.
Further, with the technique disclosed in Tokukai 2001-86354, even if an image forming medium contains a material that actively emits light energy upon receipt of the irradiation of light such as fluorescence, it is possible to acquire an image that is recognized as the one similar to a case where a picture is taken under a desired light source.
However, with the technique of Tokukai 2002-94814, it is only possible to reduce the color-unmatch with respect only to specific types of originals that use specific color materials. Namely, in the technique of Tokukai 2002-94814, the color material of the original is estimated, and then the color of the original is corrected in accordance with the type of the original, i.e., whether the original is of photographic, electrophotographic, or ink-jet. As such, with this technique, it is only possible to reduce the color-unmatch only in the case where the original is of photographic using a pre-set color material, electrophotographic, or inkjet. Thus, it is not possible to reduce the color-unmatch based on a color material other than the pre-set color material, and based on the factor other than the type of the original. Further, even in the case of the pre-set color material and the type of the original, it is not possible to correctly recognize whether or not the color-unmatch occurs if the original does not have a predetermined color.
As described above, the technique disclosed in Tokukai 2002-94814 can carry out a color correction with respect only to specified color materials. This is due to the fact that the input is 3-band input that differs from visual perception characteristics. In other words, because the color-unmatch is reduced with respect to specific color materials while the 3-band input is not processed, the technique of Tokukai 2002-94814 requires the judgment as to whether a color material is the specified one. Thus, it is possible to suppress the color-unmatch with respect only to the limited type of color materials, thereby causing a problem of lowering the accuracy of the color correction.
Further, the color-unmatch is not liable to occur in the method for estimating the spectral reflectance using the multiband camera (CCD) as disclosed in Tokukai 2001-86354. But, calculated amount for estimating the spectral reflectance becomes large. This causes the problem of requiring the image processing device to have higher performance, thereby remarkably increasing the price of the image processing device.